Method for coating metal objects with aluminum



United States Patent Office Patented Nov. 29, 1966 3,288,689 METHOD FOR COATKNG METAL OBJECTS WITH ALUMINUM Kuninori Aznma, Nara-ken, Japan, assignor to Mats!!- shita Electric Industrial Co., Ltd., Osaka, Japan, a

corporation of Japan No Drawing. Filed Feb. 27, 1963, Ser. No. 261,528

Claims priority, application Japan, Mar. 1, 1962, 37/8,063; Nov. 12, 1962, 37/51,160 5 Claims. (Cl. 204-29) This invention relates to a method. for coating metal with aluminum by use of anhydrous fused salt, and more particularly to an improved method involving electroplating with aluminum by use of anhydrous fused salt wherein the objects to be plated, e.g. steel band or steel wire are passed through a bath containing water-soluble additives and dried before electroplating.

It is known to plate aluminum on various metal objects, e.g. steel wire from an anhydrous fused salt electrolyte; namely, the double salt of aluminum chloride and alkali metal chloride. It is also known to add alkali metal halide e.g. sodium fluoride or potassium fluoride as an additive and fuse together.

Prior to this application the inventor filed US. patent application Serial Number 195,422, now Patent No. 3,236,751, in which he provided an anhydrous fused salt electrolyte, namely, the double salt of aluminum chloride and alkaline earth metal chloride e.g. barium chloride or strontium chloride and, as an additive, alkali metal borofiuoride eg sodium borofiuoride or potassium borofluoride.

The most diflicult problem of electroplating of aluminum by the anhydrous fused salt method is that of metal fog formed on the aluminum anode by an increase of bath temperature of the fused salt and current density.

When this fog appears while plating, it adversely affects current efliciency and liberated fine particles of aluminum grow into wood-branch-like crystal on the plating surface which prevents the coating from growing thicker, or liberated fine particles of aluminum become sponge-like floatings which adhere on the plating surface, spoil it and become an origin of corrosion.

Furthermore, when electroplating with the fused salt is continued, partial consumption of electrolyte components by evaporation which is inevitable in keeping the fused salt at the temperature about 160 C., or aging of electrolyte components are induced, and thereby current density suddenly decreases and becomes a cause of bad plating.

These defects have been a serious obstacle in the continuous plating on a commercial scale of eg steel band and the like.

The present invention provides a method in which a bath containing water-soluble salt is arranged before the electroplating bath and by which the obstacle to continuous plating operation i.e. aging of electrolyte bath is removed and the production of beautiful, anticor-rosive plating of aluminum is thereby possible.

Though the method of this invention is not restricted by the composition of the electroplating bath, the following are suitable compositions by weight which serve the purpose.

(a) Percent Aluminum chloride 60 to 85 Alkaline earth metal chloride to 25 Alkali metal borofiuoride Less than 0.5

(b) Percent Aluminum chloride 60 to 85 Sodium chloride 10 to 20 Alkaline earth metal chloride 10 to 35 Alkaline earth metal fluoride 0.3 to 5 Aluminum fluoride 0.5 to 10 The water-soluble additive in the bath is composed of aluminum chloride and either alkaline earth metal chloride or alkali metal chloride and metal fluoride, and a composition which is effective for the purpose is as follows wherein percentages are by weight.

Percent Aluminum chloride 10 to 25 Alkaline earth metal chloride or alkali metal chloride 0.1 to 10 Alkaline earth metal fluoride or metal fluoride 0.1 to 5 Among the above compositions, that are shown hereunder is preferable for the water-soluble additive.

Percent Aluminum chloride 10 to 20 Alkaline earth metal chloride 0.5 to 10 Alkaline earth metal fluoride 0.5 to 2 Aluminum fluoride 0.5 to 5 When the water-soluble additive of the (d) composition is use-d, a plating bath of the (b) composition is effective.

As for the temperature of the bath containing the watersoluble additive, ordinary room temperature is employed, however, in order to effect the drying process in the successive step smoothly, a temperature slightly higher than room temeprature is favored. If the temperature is too high, metallic compounds existing in the watersoluble additives are converted into oxides, which is undesirable. The preferred range is between room temperature and C. When the temperature of the process is low, drying cannot be con-ducted sufficiently, and when the temperature is too high, aluminum chloride is vaporized, causing the formation of metallic oxides. Therefore, a temperature of 2001-10 C. is preferable.

As an alkaline earth metal of the plating bath, barium is preferable, because its potentional becomes considerably more basic, sometimes more basic than that of alkali metal. Further, when the electroplating is conducted with a bath containing alkaline earth metal chloride and fluoride, appearance of metal fog is much more greatlyreduced than with any other fused salt plating bath of the same temperature and using the same current density. Current efficiency is high, the plating surface is beautiful, coating is thick and the anti-corrosive property is excellent. It is easy to obtain a coating having a thickness of 7;]. to 13,14. Moreover, it is possible to make the current density greater than that obtained by any other electrolyte, and accordingly it is possible to increase the line speed. of continuous plating of e.g. band steel and boost up production efficiency.

The melting point of the plating bath varies according to the proportion of each compoent, but generally it is about C. Though it is preferable to make the plating temperature as low as possible, it is diflicult to obtain a bright plating surface at a temperature below C. because of low plating efficiency, and at a temperature higher than C. formation of metal fog is increased. Therefore, a plating temperature of 1601-5 C. is most suitable.

The following examples are given to show the embodiments of this invention.

Example I As an electroplating bath, the following composition is used.

Percent Aluminum chloride 78 Barium chloride 13 Strontium chloride 8.6 Sodium borofluoride 0.4

Also as a water-soluble additive bath the composition as shown hereunder is used.

Percent Aluminum chloride 10 to 25 Aluminum fluoride 0.1 to 1 Lead fluoride 0.1 to 1 Barium chloride 0.1 to 1 Example II An electroplating bath having the following composition is used,

An electroplating bat-h having the following composition is used.

Percent Aluminum chloride 70 Sodium chloride 15 Barium chloride 11 Aluminum fluoride 4 A water-soluble additive bath having the following composition is used.

Percent Aluminum chloride 1O Barium chloride Aluminum fluoride 1 3 Aluminum fluoride is used after converting it into the hydrate of aluminum fluoride A1F3'3 H2O.

Example IV An electroplating bath having the following composition is used.

Percent Aluminum chloride 70 Sodium chloride 15 Barium chloride 12 Barium fluoride 3 A water-soluble additive bath having the following composition is used.

Percent Aluminum chloride Barium chloride 5 Barium fluoride 4 In each example steel band was passed through a room temperature solution of a water-soluble additive bath and dried at 200 C. to deposit the water-soluble additive on the surface of the steel band. Next the steel band was immersed, as a cathode, in an anhydrous fused salt electrolyte bath at the temperature of 160 C., against an anode of pure aluminum, D.C. current was passed at the density current of 3 -a./dm. After 5 minutes of plating, 6,11. and after 10 minutes 10p. of aluminum coating were obtained. In each casethe plated coating was smooth and very lustrous. It adherence was excellent and current efliciency was high, and continuous operation was possible.

In summary, this invention provided an improved method of continuous electroplating of aluminum by the anhydrous fused salt method which is characterized in dipping the object to be plated in a bath of water-soluble additive consisting essentially of aluminium chloride before dipping in the anhydrous fused salt plating bath, drying the above object, whereby the water-soluble additive deposits on the surface and then dipping the same object in the anhydrous fused salt plating bath. This method affords a beautiful aluminum coating continuously and is very valuable in commercial practice.

What is claimed is:

1. In the method for coating ferrous objects with aluminum using the anhydrous fused salt electroplating process, the improvement consisting first of immersing the object to be plated in an aqueous solution of watersoluble additives consisting of aluminum chloride, at least one chloride selected from the group consisting of alkali metal chloride and alkaline earth metal chloride and at least one fluoride selected from the group consisting of alkaline earth metal fluoride, aluminum fluoride and lead fluoride, said water-soluble additives consisting essentially of about 10% to 25% of aluminum chloride, about 0.1% to 10% of at least one chloride selected from the group consisting of alkaline earth metal chloride and alkali metal chloride and about 0.1% to 5% of at least one fluoride selected from the group consisting of alkaline earth metal fluoride, aluminum fluoride and lead fluoride, then drying the object whereby the surface is coated with the additives of said aqueous solution and then introducing the object into the electroplating bath.

2. Method according to claim 1 wherein the temperature of the aqueous solution ranges from room temperature to C.

3. Method according to claim 1 wherein the object References Cited by the Examiner -UNITED STATES PATENTS 2,315,725 4/1943 Moller 117114 2,709,154 5/1955 Hansgirg 204--39 2,971,899 2/1961 Hanink et al. 20429 3,007,854 11/1961 Smith et a1. 20439 X 3,048,497 8/1962 Moller 20439 X JOHN W. MACK, Primary Examiner.

G. KA-PLAN, Assistant Examiner. 

1. IN THE METHOD FOR COATING FERROUS OBJECTS WITH ALUMINUM USING THE ANHYDROUS FUSED SALT ELECTROPLASTING PROCESS, THE IMPROVEMENT CONSISTING FIRST OF IMMERSING THE OBJECT TO BE PLATED IN AN AQUEOUS SOLUTION OF WATERSOLUBLE ADDITIVES CONSISTING OF ALUMINUM CHLORIDE, AT LEAST ONE CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL CHLORIDE AND ALKALI EARTH METAL CHLORIDE AND AT LEAST ONE FLUORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALINE EARTH METAL FLURODE, ALUMINUM FLUORIDE AND LEAD FLUROIDE, SAID WATER-SOLUBLE ADDITIVES CONSISTING ESSENTIALLY OF ABOUT 10% TO 25% OF ALUMINUM CHLORIDE, ABOUT 0.1% TO 10% OF AT LEAST ON CHLORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALINE EARTH METAL CHLORIDE AND ALKALI METAL CHLORIDE AND ABOUT 0.1% TO 5% OF AT LEAST ONE FLUORIDE SELECTED FROM THE GROUP CONSISTING OF ALKALINE EARTH METAL FLUORIDE, ALUMINUM FLUORIDE AND LEAD FLUORIDE, THEN DRYING THE OBJECT WHEREBY THE SURFACE IS COATED WITH THE ADDITIVES OF SAID AQUEOUS SOLUTION AND THEN INTRODUCING THE OBJECT INTO THE ELETROPLATING BATH. 